The present invention relates to a base catalyzed process for preparing polyorganosiloxane polymers with controlled low-levels of hydroxy substitution. Residual hydroxyl radicals substituted on polyorganosiloxane polymers provide reactive sites on the polymer chain that can cause polymer chain extension. This polymer chain extension results in viscosity shifts of the fluid and consequently reduced shelf life. In addition, the presence of these residual reactive sites are important to the physical characteristics and performance of the final product. Therefore, it is not only important that hydroxy substitution be low on polyorganosiloxane polymers, it is also important to be able to control the level of these reactive sites within specified limits.
Current production of polyorganosiloxane polymers, for example, polydimethylsiloxane, is a multi-step process. In a typical process, a first step involves hydrolyzing dimethyldichlorosilane and subsequent condensation of the hydrolysate to form an equilibrium mixture of low molecular weight permethylcyclosiloxanes and low molecular weight dimethylsiloxane linears. This condensation product is exhaustively washed to hydrolyze and remove chloride from the polymer. The wash is necessary to provide acceptably low levels of chloride in the final product.
The condensation products, free from chloride, are then run through a polymerization process to create high molecular weight polydimethylsiloxane fluids. Current processes require highly elevated temperatures and accordingly special equipment to run the processes. Considerable variability is experienced in the residual amount of hydroxy substitution of the polydimethylsiloxane fluid.
Therefore, it is an objective of the present invention to provide a polyorganosiloxane fluid with low-levels of hydroxy substitution. A second objective is to provide a process whereby the level of hydroxy substitution of the product polyorganosiloxane fluid can be controlled. A third objective is to provide a process whereby out of specification, polyorganosiloxane fluids can be re-processed with minimal detrimental alterations of the fluid.
Prior publications teach that bases can be used to catalyze the condensation reaction of low molecular weight polyorganosiloxanes and that water can be used to regulate the chain length of the polymer. However, the processes taught in the prior publications do not address the control of hydroxy substitution and generally resulted in polyorganosiloxane fluids with high residual levels of hydroxy substitution.
Voronkov et al., The Siloxane Bond, Consultants Bureau, New York, 1978, page 159/198, discusses possible base catalysts in polymerization of cyclic polyorganosiloxanes. These base catalysts include any Group IA alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, cesium hydroxide, and potassium hydroxide; alcoholates, phenolates, silanolates, siloxanolates, and mercaptides of the Group IA alkali metals; quaternary ammonium and quaternary phosphonium bases and their siloxanolates; organolithium, organosodium, and organopotassium compounds; compounds of general formula RM, where R is a hydrocarbon radical, and M is an alkali metal atom: R.sub.4 N.sup.+, and R.sub.4 P.sup.+, where R is as previously described. Voronokov et al. teaches that chain length and of influencing the activity of some of the base catalysts. water can be used as a source of controlling the polymer
U.S. Pat. No. 4,250,290, issued Feb. 10, 1981, discloses a process for the continuous manufacturing of diorganopolysiloxanes wherein it is possible to control the chain length by adding an appropriate amount of water as an endblocker. The viscosity of the polymer is monitored to determine the amount of water to be added to the reactor. A suitable base catalyst is used to facilitate the polymerization process; the base catalyst is selected from a group consisting of cesium hydroxide, potassium hydroxide, sodium hydroxide, cesium silanolate, potassium silanolate, and sodium silanolate. The polydiorganosiloxane products are described as silanol terminated. Peterson also claims the continuous polymerization of triorganosilyldiorganopolysiloxane polymers as a similar process, utilizing a low molecular weight triorganosilyldiorganopolysiloxane as a chain stopper in the absence of water.
Buchner et al., U.S. Pat. No. 4,128,568, issued Dec. 5, 1978, discusses a process for the continuous preparation of polydiorganosiloxanes with viscosities of 10 to several million centipoise, wherein an endblocker is used to regulate the viscosity of the product polymer, and the polymerization is facilitated by either an acid or base catalyst. Buchner reports using short-chained polydimethylsiloxanes as endblocker to control the viscosity of the product polymer.
Herberg et al.. U.S. Pat. No. 4,551,515, issued Nov. 5, 1985, discloses a process for the continuous manufacturing of silicon polymer, including high viscosity silicon polymers known as silicon gums. One or more cyclopolysiloxane monomers are mixed with one or more endblockers and preheated prior to the addition of a catalyst for facilitating the polymerization reaction. Endblockers and base catalysts described by Herberg et al. are previously disclosed by Peterson. Water is reported to terminate or endblock polydiorganosiloxane polymer with silanol groups and thus reduce the viscosity of the polymer and change its chemical nature; it is therefore removed from the reactants.
Hansen et al., co-pending U.S. patent application No. 612,655, filed Nov. 14. 1990, describes a process by which hydrogen chloride catalyzes the condensation of chloride and hydroxy end-substituted polyorganosiloxanes, in the presence of a triorganosilyl endblocker for controlling the polymer chain length. The hydrogen chloride concentration is varied within a range of about 10 to 30 weight percent to control the level of hydroxy substitution of product polyorganosiloxanes.
The inventors have discovered that by using triorganosilyl groups as endblocker to control polymer length during the base catalyzed condensation of polyorganosiloxanes, the partial pressure of water and temperature of the process can be varied within defined ranges to control the level of hydroxy substitution on the ends of polyorganosiloxane polymers.
The low-hydroxy substituted polyorganosiloxanes formed by the present method are useful, for example, as antiflatulents, antifoam compounds, and as intermediates in the formulation of sealants. The ability to control the level of hydroxy substitution, even when present at low levels, is important in producing stable formulations and formulations with predictable chemical reactivity.